Data storage devices of the type known as "Winchester" or "hard" disc drives are typically provided with a plurality of rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. A controllably positionable actuator is disposed adjacent the discs, the actuator including a plurality of heads which are used during write and read operations to magnetically store and retrieve user data from tracks defined on the disc surfaces.
A closed loop servo system is used to control the position of the heads with respect to the tracks on the disc. More particularly, the actuator typically includes a coil of a voice coil motor (VCM) so that currents applied to the coil by the servo system cause the heads to move relative to the tracks in a controlled manner. A read/write channel, responsive to the heads, cooperates with an interface circuit to control the transfer of user data between the discs and a host computer in which the disc drive is mounted.
As will be recognized, consumer demand for disc drives with ever increasing data storage capacities and transfer rates has led to continual advances in the disc drive art. One such advancement is the use of magneto-resistive (MR) heads, each of which typically includes an MR element having a changed electrical resistance in the presence of a magnetic field of a particular orientation. Thus, during a disc drive read operation a bias current is passed through the MR element and the data stored on the corresponding track are detected as a function of changes in voltage across the MR element.
Another advancement in the disc drive art facilitating greater data storage capacities has been the continued reduction in the flying heights of the heads above the recording media. However, bringing the heads in closer proximity to the discs increases the sensitivity of the disc drive to the effects of anomalous conditions caused by defects associated with the media, especially with the use of MR heads. Particularly, irregularities on the surface of the discs can be large enough to physically contact the MR element of the heads as the discs rotate under the heads. Such contact, while of very short time duration, results in frictional heating of the MR element. The change of temperature brought about by the contact correspondingly produces a change in the resistance of the MR element. Such events are known as thermal asperities, or TAs, and can significantly distort the readback signal generated by the head. A TA event is typically characterized by a sudden increase in read signal amplitude, followed by a relatively long falling edge due to the heat dissipation time constant of the MR head.
Small "hills" and "valleys" in the disc surfaces can also induce TA events even without physical contact occurring between the MR element and the disc surface. Because the bias current applied to the MR element results in heating of the MR element, a thermal equilibrium is established in which the generated heat in the MR element is continually dissipated from the MR element through other elements of the head assembly and, to a lesser extent, across the air bearing supporting the head above the disc. Thus, disc surface variations that change the spacing between the MR element and the disc can induce attendant changes in the heat dissipation characteristics of the head, resulting in localized distortion in the readback signal obtained from the head.
TAs found in disc drives using currently available media are of a size which can span a significant number of bytes; for example, in a disc drive having a data transfer rate of 200 megabits per second (Mbits/sec), uncompensated thermal asperities can last from 1 to 5 microseconds, distorting from about 25 to 125 bytes of data. Further, it will be recognized that TAs can grow over time due to factors such as contamination and corrosion of the disc surfaces, which can significantly degrade the capabilities of a disc drive to reliably store and retrieve user data over the operational life of the drive.
Compensation for the effects of thermal asperities is typically provided through the use of error detection and correction circuitry of the read/write channel, provided that the number of affected bytes is within the correction capability of the circuitry. Efforts have also been made in the prior art to filter the read signal upon the occurrence of a TA to reduce the time constant of the falling edge of the TA in order to reduce the number of bytes affected by the TA.
However, limitations have been encountered with such prior art approaches. Firstly, the selective introduction of additional filtering in response to TAs typically increases a disc drive read error rate, due to the corresponding movement of poles within read circuitry of the read/write channel. Secondly, the filtering is typically provided for a predetermined length of time, so that error rates are also increased for bytes immediately following the TA and not otherwise affected by the TA.
Thirdly, such filtering is usually programmable to compensate for various factors, including the characteristics of each detected TA as well as to compensate for variations in data transfer rates across the radii of the discs. Such programmability increases the complexity of the read circuit and the associated overhead for devices controlling the operation of the read circuit. Finally, such filtering ultimately limits the maximum data transfer rate of a disc drive, in that greater numbers of bytes are affected by a TA as the transfer rate increases, whereas the number of erroneous bytes that can be corrected by the error detection and correction circuitry generally remains constant in successive generations of drives. Hence, there is a maximum transfer rate beyond which the number of bytes affected by a TA event will exceed the error correction capability of the read circuit, even with the selective use of filtering to minimize the number of erroneous bytes.
Accordingly, there is a need for an improved, simplified approach to compensating for the effects of thermal asperities that can significantly reduce the number of affected bytes in a read signal and can accommodate ever increasing disc drive transfer rates.